Optical techniques based on the Near-infrared spectral window have made significant progress in biomedical research in recent years. The relative low absorption and low scattering in the 600-1000 nm spectral range allow detection of photons that have traveled through several centimeters of biological tissue [1]. Coupled with accurate models of light propagation, NIR techniques enable imaging of deep tissue with boundary measurements using non-ionizing, low dose radiation.
The interest in NIR techniques is fueled by the ability of the techniques to monitor functional tissue parameter such as oxy- and deoxy-hemoglobin [2] and the development of appropriate low cost instrumentation. Based on these qualities, NIR optical imaging is expected to play a key role in breast cancer detection, characterization [3, 4, 5, 6, 7, 8] and monitoring through therapy [9]; brain functional imaging [10, 11, 12, 13] and stroke monitoring [14, 15]; muscle physiological and peripheral vascular disease imaging [16, 17]. For all these applications, NIR techniques rely on endogenous contrast such as tissue hemodynamics. Another potential application of NIR technique is to monitor exogenous contrast. Especially, we see the emergence of an optical molecular imaging field that bears great promises in clinical applications [18].
NIR fluorescence optical imaging is rapidly evolving as a new modality to monitor functional data in either human or animal tissue. The developments of new contrast agents that target specific molecular events [19, 20, 21] are particularly promising. By specifically binding [22, 23] or being activated in tumors [24], detection can be achieved in the early stages of molecular changes prior to structural modification [25]. Moreover, the endogenous fluorescence in the NIR spectral window is weak leading to exquisite fluorescence sensitivity.
NIR molecular imaging is still confined to small animal models [26] and the translation to human imaging is foreseen as imminent. However, the technical problems encountered in imaging large tissues are challenging. Besides sensitive instrumentation [27], robust and accurate models for fluorescent light propagation are needed. Tomographic algorithms in the continuous mode [28] and in the frequency domain [29, 30] have been proposed. Both numerical and analytical models exist and have been applied successfully to experimental data. However, there is a need for the time-domain algorithms.